JPWO2015111458A1 - Grating coupler - Google Patents

Abstract

The one-dimensional grating coupler has a problem that the structure is large, and the arc-shaped two-dimensional grating coupler has a problem that it is difficult to design. The optical branching element 30 receives light from the input end via the optical waveguide 40 and branches and outputs the light to the tapered waveguides 20 and 21 from the two output ends. The grating element 10 is a one-dimensional diffraction grating and emits light propagating through the tapered waveguides 20 and 21 to the outside. By splitting the light, the length of the tapered waveguide can be halved compared to the conventional one-dimensional periodic grating coupler. Even if the length of the optical branch is added to the length of the tapered waveguide, The overall length of the grating coupler of the one-dimensional period grating coupler can be greatly reduced compared to the length of the tapered waveguide, and the structure can be downsized.

Description

  The present invention relates to a grating coupler.

  In order to input / output optical signals to / from an optical circuit on a substrate composed of an optical waveguide, the function to emit the light propagating through the optical waveguide to the outside of the optical waveguide or to allow external light to enter the optical waveguide is provided. There is something I want to do. A grating coupler is an optical input / output device that can be used for such a purpose. The propagation light of an optical waveguide on a substrate is incident on an end face of an optical fiber close to the surface of the substrate, or vice versa. Can be incident on the optical waveguide.

  FIG. 7A is a schematic cross-sectional view of a grating portion as an example of a grating coupler having a configuration for emitting guided light to the outside of the optical waveguide. Hereinafter, the structure and function of the grating coupler will be briefly described with reference to FIG. 7A.

  The grating coupler shown in FIG. 7A has an action of receiving an optical signal through an optical waveguide and converting an optical axis by diffraction. The cross-sectional structure illustrated in FIG. 7A includes a BOX (Burried Oxide) layer 702, a core layer 714 having a refractive index higher than that of the BOX layer 702, and an upper cladding (Overclad) having a refractive index comparable to that of the BOX layer 702. ) Layer 716 is laminated on the substrate 701 in this order, and the core layer 714 has a diffraction grating formed thereon. In this way, the grating coupler enables optical path conversion only by forming a diffraction grating on the core layer 714, for example.

  Hereinafter, the operation principle of the grating coupler for emitting the guided light to the outside of the optical waveguide will be outlined.

In the core layer 714 of the grating coupler 700 shown in FIG. 7A, a diffraction grating that is periodic in the propagation direction (z direction) of the guided light and has irregularities in the thickness direction (x direction) of the waveguide is formed. . When such a diffraction grating is formed, part of the guided light is diffracted to generate radiated light. The guided light and radiated light satisfy the phase matching condition in the propagation direction (z direction). That is, the propagation constants of the guided light beta _0, if the z-direction propagation constant of the emitted light and beta _q, the phase matching condition is β _{q =} β _{0 +} qK. Where Λ is the period of the diffraction grating, K = 2π / Λ, and q is a value corresponding to the order of the emitted light (0, ± 1, ± 2,...).

In this case, emitted light output angle θ of the relative to the normal of the diffraction grating, the wavelength in vacuum of light lambda, the effective refractive index of the waveguide of N, n _c the refractive index of the upper cladding layer, lambda of the diffraction grating Assuming a period, it can be obtained from the formula n _c sin θ = N + qλ / Λ.

In general, the effective refractive index of the waveguide N is takes a value between the refractive index and the refractive index of the cladding of the core, have a relationship of N> n _c, the emission of light occurs satisfies q ≦ -1 Limited to order q. Furthermore, considering that the diffraction efficiency with a smaller diffraction order is higher, the power distribution ratio to the emitted light can be maximized when the emitted light satisfying q = −1 is used. In addition to the above-described primary radiation light and the like, there are simultaneously return light P _{ref in the} waveguide direction, core layer transmitted light P _trans , and radiation light P _down toward the substrate 701 via the BOX layer 702.

Here, the emission angle θ can be arbitrarily designed according to the period Λ, width w, depth d, and thickness D of the optical waveguide shown in FIG. 7A. An example of numerical ranges of the period Λ, the width w, the depth d, and the thickness D of the optical waveguide for light having a wavelength of 1.3 μm to 1.6 μm in vacuum is as follows.
Λ: 530 to 550 nm
FF (= 1-w / Λ): 0.3 to 0.6
d: 60 to 80 nm
D: 180-220 nm

  As described above, the case where guided light is emitted outside the optical waveguide by the grating coupler has been described. However, it has been widely performed that external light is incident on the optical waveguide by such a grating coupler.

  As described above, the grating coupler is basically based on a structure in which a diffraction grating is formed on the surface of the core. However, in the process of improving performance, structures having various characteristics have been proposed. For example, the conventional most basic grating coupler has a one-dimensional grating (grating) in which irregularities are periodically arranged in the length direction of the waveguide, and the grating is linear in the transverse width direction of the waveguide. It is uniform. If the grating is uniform in the width direction of the waveguide, the structure in the width direction can be ignored and approximated to a two-dimensional structure in the length direction and the thickness direction. Therefore, when the radiated light from the grating coupler is obtained by numerical calculation, the calculation scale can be reduced, and as a result, there is an advantage that a grating coupler with good optical coupling efficiency can be designed in a short time.

  On the other hand, since the lateral width of the grating portion used for optical coupling with the optical fiber is often 10 times or more the width of the optical waveguide on the substrate, a long and gentle taper is formed between the grating portion and the optical waveguide. It was necessary to insert a waveguide. This was to suppress the disturbance of the wavefront of the propagating light at the connecting portion between the grating portion and the optical waveguide, but as a result, the overall structure of the optical input / output element combining the grating portion and the tapered waveguide was increased, It was difficult to incorporate into a high-density optical circuit with few gaps (Non-Patent Document 1). For example, according to our study, as shown in FIG. 7B, in the conventional grating coupler having a one-dimensional period, it is necessary to provide a tapered waveguide 720 having a length of about 15 times or more the width of the grating portion 710. If the width of the grating portion 710 is 20 μm, it is necessary to provide a tapered waveguide 720 having a length of about 300 μm at the minimum.

  Therefore, as a new conventional technique, a grating coupler in which an arc-shaped grating is formed in a tapered waveguide having a large divergence angle has been devised (Patent Documents 1 and 2). When a tapered waveguide having a large divergence angle is used, the wavefront of propagating light spreads in an arc shape, unlike the case of using a long and gentle tapered waveguide. In order to keep parallel to the wavefront, the grating was also formed in an arc shape. As described above, by allowing the arc-shaped wavefront to spread, even when optically coupled to an optical fiber, the maximum width of the grating portion is about 50 μm, and the combined length of the waveguide and the grating portion is also increased. Thus, the overall structure including the grating portion and the tapered waveguide can be reduced in size (see FIG. 7C).

  However, since the grating has an arc shape, the grating is not uniform in the transverse width direction of the waveguide, making it impossible to perform two-dimensional approximation in numerical calculations. For this reason, in order to know the characteristics of the arc-shaped grating coupler, it becomes necessary to perform three-dimensional calculation that requires enormous computer power and time, or to repeatedly perform time-consuming trial manufacture and verification by experiment. A new problem has arisen that it takes a lot of time to adjust the detailed structure in search of good properties.

US Pat. No. 7,245,803 US Pat. No. 7,260,289

D. Taillaert et al, “An Out-of-Plane Grating Coupler for Efficient Butt-Coupling Between Compact Planar Waveguides and Single-Mode Fibers,” IEEE JOURNAL OF QUANTUM ELECTRONICS, Vol. 38, No. 7, JULY 2002, p. 949-955,

  In order to solve the above-mentioned problem of the large structure of the linear grating coupler and the difficulty of design of the arc-shaped grating coupler, it is compact and the structure can be optimized at the same time. It is a problem to provide an easy grating coupler.

  In order to solve the above-mentioned problems, the grating coupler of the present invention has a special structure in which the entire structure for expressing the function as an optical input / output element has a particular relationship between a one-dimensional grating, a tapered waveguide, and an optical branching element. It is the structure combined with the group, The embodiment is as follows generally. In the following, the names of “input end” and “output end” are assigned assuming that light input from the waveguide is output from the grating coupler to the substrate surface side, but the light propagation direction is opposite. It should be noted that the assignment of names is reversed.

  In the basic form of the grating coupler of the present invention, the grating coupler includes a grating portion, a tapered waveguide portion, and an optical branching portion, and the grating portion is one grating element or a plurality of gratings arranged adjacently in parallel. Each of the grating elements includes a one-dimensional grating, the one-dimensional gratings of the plurality of grating elements have periodicity in the same or substantially the same direction, and the tapered waveguide portion includes the grating element. A plurality of tapered waveguides, wherein the optical branching section has one input end and at least a plurality of output ends equal to the number of the tapered waveguides, and each of the grating elements includes a tapered waveguide. One end of the wide side is connected to one or more in parallel, and each of the tapered waveguides The narrow side of the end width, one of a plurality of output ends of the optical branching section is connected.

  The operation of this basic mode is, for example, in the aspect of emitting light from the grating unit to the outside, the light incident on the input end of the optical branching unit is branched at the optical branching unit, and each branched light is transmitted from the optical branching unit. It is output from a plurality of output ends, is input to one or a plurality of grating elements via a tapered waveguide connected to each of the plurality of output ends, the optical path is changed by the grating elements, and is emitted to the outside. Is.

  In this basic mode, a plurality of parallel tapered waveguides are used to input light to a grating having the same width as a conventional one-dimensional periodic grating coupler or to adjacent gratings having the same width in total. Therefore, when compared with the conventional one-dimensional periodic grating coupler in which only one tapered waveguide is provided without providing an optical branching portion, each taper is inversely proportional to the number of branches, that is, the number of tapered waveguides. The waveguide is reduced. The effect that the entire length of the grating coupler can be made shorter than the total length of a conventional grating coupler having a one-dimensional period if the length shortened with the reduction is larger than the length of the newly added optical branching portion. Produce. On the other hand, since the grating portion remains a one-dimensional grating, the ease of designing the light incident / exit characteristics is maintained. Needless to say, an aspect in which light is incident on the grating portion from the outside also has the same effect.

  In one form of the grating coupler of the present invention, in the basic form, among the one-dimensional gratings of all the grating elements, the grating elements adjacent to each other over the range of the length of the one-dimensional grating that is shortest in the direction having periodicity. In the meantime, each lattice position of the one-dimensional lattice is arranged in a direction orthogonal to the direction in which the one-dimensional lattice has periodicity. By adopting such a configuration, it is possible to cause the gratings of the grating elements to act on the propagation light incident through the tapered waveguides with the same phase relationship. As a result, the phase of the diffracted light from each part of the grating part is aligned, and the radiated light having an appropriate intensity distribution can be easily achieved.

  In another mode of the grating coupler of the present invention, the optical branching unit in each of the above modes is configured by a multimode interferometer. By adopting such a configuration, it is possible to easily configure an optical branching portion that is shorter than the length of the tapered waveguide shortened by division. If there is a margin in the optical branching ratio and the allowable optical loss, the optical branching unit may be configured using a Y-branch or a directional coupler instead of the multimode interferometer.

  In another embodiment of the grating coupler of the present invention, the grating coupler of each of the above embodiments is configured to have an axis of line symmetry. By adopting such a configuration, it is possible to easily achieve an appropriate light intensity distribution excellent in symmetry in the width direction in the grating portion.

  In another form of the grating coupler of the present invention, in each of the above forms, the grating section includes three or more grating elements, and the width of a direction perpendicular to the direction in which the one-dimensional grating has periodicity is 2 There are more than one type. By adopting such a configuration, for example, when the width of one grating element is made half of the width of another grating element, the light intensity can be increased by a factor of only the radiated light from that grating element. By using such a method, the light intensity distribution in the width direction of the grating portion, that is, the shape of the mode field can be adjusted more finely. Therefore, for example, even when there are a plurality of types of optical fibers that are optical coupling partners and their mode fields are different, it is possible to perform an optimization design in the width direction in accordance with each mode field. .

In another embodiment of the grating coupler of the present invention, each of the above embodiments includes a plurality of grating elements having different distances from the connection position with the tapered waveguide to the start position of the grating. By adopting such a configuration, the phases of the return light P _ref (see FIG. 7A) reflected at the grating start position of each grating element and returning to the waveguide direction can be shifted from each other. As a result, it is possible to use the action of canceling each individual return light in the process of backflowing through the optical branching portion and being combined, and it is possible for the grating coupler as a whole to reduce the reflected return light. become.

  In another embodiment of the grating coupler of the present invention, the number of grating elements is particularly one in the above-described four embodiments. By adopting such a configuration, the structure of the grating portion optimized for the conventional grating coupler having one one-dimensional grating can be used as it is, and it is significantly shorter despite a short design time. A grating coupler can be manufactured.

  In another embodiment of the grating coupler of the present invention, in each of the above embodiments, at least one of the period of the one-dimensional grating of each grating element included in the grating portion, the depth of the unevenness, the layer structure, and the refractive index of the medium is It is configured to be modulated in a direction having periodicity. By adopting such a configuration, it is possible to realize a lattice mode that is not uniform in the direction having periodicity in the grating portion, and to easily optimize the shape of the mode field.

  According to the present invention, it is possible to provide a grating coupler that is small in size and can be easily designed in accordance with a mode field to which light is coupled at the same time.

FIG. 1 is a schematic plan view showing the configuration of an embodiment of the grating coupler of the present invention. FIG. 2 is a schematic plan view showing the configuration of another embodiment of the grating coupler of the present invention. FIG. 3 is a schematic plan view showing the configuration of another embodiment of the grating coupler of the present invention. FIG. 4 is a schematic plan view showing the configuration of another embodiment of the grating coupler of the present invention. FIG. 5 is a schematic plan view showing the configuration of another embodiment of the grating coupler of the present invention. FIG. 6 is a schematic plan view showing the configuration of another embodiment of the grating coupler of the present invention. FIG. 7A is a schematic cross-sectional view illustrating an exemplary configuration of a grating coupler. FIG. 7B is a schematic plan view of a grating coupler having a one-dimensional period. FIG. 7C is a schematic plan view of a grating coupler in which an arcuate grating is formed in a tapered waveguide having a large divergence angle.

  Hereinafter, embodiments of the grating coupler of the present invention will be described with reference to the drawings. However, the technical scope of the present invention is not limited by these embodiments, and should be interpreted based on the description of the claims. Strictly speaking, the “structure” of each part of the grating coupler is a “core structure”, and it goes without saying that the cladding surrounds the periphery. In the following description, a mode in which light is emitted from the grating unit to the outside will be mainly described. However, a mode in which light is incident on the grating unit from the outside can also be realized with the same configuration.

  FIG. 1 is a schematic plan view showing the configuration of an embodiment of the grating coupler of the present invention. As shown in FIG. 1, the grating coupler of the present embodiment forms a grating section 100 with one grating element 10 and a tapered waveguide section 200 composed of two tapered waveguides 20 and 21 arranged in parallel. An optical branching unit 300 including one optical branching element 30 is provided, which is formed on a semiconductor substrate (not shown) (for example, a silicon substrate).

  The optical branching element 30 receives light from the input end via the optical waveguide 40 and branches and outputs the light to the tapered waveguides 20 and 21 from the two output ends. The optical branching element 30 is composed of, for example, a 1-input 2-output multimode interferometer (MMI) having a core made of Si and a clad made of a silicon oxide film. In general, a multimode interferometer has a self-convergence effect that is periodically reproduced in the process of propagation of a light field at the incident end through the multimode interference waveguide, and is dependent on the width of the multimode interference waveguide. One or more converging fields can be obtained at a specific distance from the determined input end. By connecting the waveguide with the convergence position as the output end, a very low loss optical branch can be configured.

  Although the length of the optical branching element 30 from the input end to the output end in the present embodiment needs to be equal to or longer than the specific distance, it can be set within about 15 μm.

  Each of the tapered waveguides 20 and 21 is a planar optical waveguide having a function of guiding light along the substrate surface, and its narrow end is connected to the output end of the optical branching element 30. The wide end is connected to the grating element 10.

  The lengths of the tapered waveguides 20 and 21 in this embodiment can be suppressed to about half of the length of the tapered waveguide of the conventional one-dimensional period grating coupler, respectively. If the length of the tapered waveguide is about 300 μm, it can be shortened to about 150 μm.

  Note that the tapered shape of the tapered waveguides 20 and 21 is not limited to a linear tapered shape, and includes, for example, a curved tapered shape such as a quadratic curve or a semi-elliptical shape.

  The optical branching section and the tapered waveguide section can be configured as a buried optical waveguide having a cross-sectional structure as shown in FIG.

  The grating element 10 included in the grating portion is a one-dimensional diffraction grating composed of periodic grooves having a predetermined interval and depth, and has a function of emitting light propagating through the tapered waveguides 20 and 21 to the outside. I have it. The functions and the like are as outlined in FIG. 7A in the [Background Art] column. The one-dimensional lattice does not need to be uniform, and may have an apodized structure in which, for example, the period and the groove depth gradually change.

  The length of the grating element 10 is substantially the same as the length of the grating portion of the conventional one-dimensional period grating coupler. If the length of the grating portion of the conventional one-dimensional period grating coupler is about 30 μm, the length is about 30 μm. It will be about. Therefore, when compared with the above assumed numerical values, the length of the conventional one-dimensional period grating coupler is about 330 μm, whereas the length of the grating coupler of this embodiment is the length of the optical branching element 30. Even if this is added, the total length can be reduced to about 195 μm, and the overall length can be greatly reduced as compared with a conventional grating coupler having a one-dimensional period.

  1 represents a line symmetry axis of the grating coupler of the present embodiment. By making the structure symmetric with respect to the symmetry axis, it is possible to guarantee that the light branched by the optical branching portion passes through the tapered waveguide portion with the same phase and is input to the grating portion with the same phase. . The light incident on the grating portion further acts on the grating with the same phase relationship, and is finally diffracted and emitted with the same phase. Thus, by making the grating coupler symmetrical, even if a branch element is included, an optically symmetric operation similar to the conventional structure can be obtained.

  However, the method for obtaining such a symmetrical operation is not limited to making the structure of the grating coupler symmetrical. Since the light emitted from the grating portion only needs to have the same phase, for example, as a result of passing through the branching portion and the taper portion, the phases of the plurality of lights incident on the grating portion are different by an integral multiple of 2π. The grating coupler may be formed asymmetrically. Specifically, as long as the phase of the radiated light is symmetric, the lengths of the two output waveguides of the optical branching element 30 may be made different, or the lengths of the tapered waveguides 20 and 21 may be made different. . If the mode field of the external optical waveguide that is optically coupled to the grating coupler is asymmetric, or if the optical coupling angle forms a finite angle with the axis of the emitted light, the emitted light from the grating coupler It should also be noted that the phase may be intentionally asymmetric.

  FIG. 2 is a schematic plan view showing the configuration of another embodiment of the grating coupler of the present invention. The difference from the embodiment of FIG. 1 is that the optical branching unit 300 has a two-stage configuration of two branching elements (consisting of three optical branching elements 30 to 32) and four output ends of the optical branching unit 300 are provided. This is the point that a tapered waveguide section 200 composed of four tapered waveguides (22 to 25) is provided. In the case of FIG. 2, a 1-input 2-output branch element is combined to form a 1-input 4-output branch. However, for example, a 1-input 4-output MMI may be used.

  The configuration and function of the grating element 10, the tapered waveguides 22 to 25, the optical branching elements 30 to 32, the optical waveguide 40, and the line symmetry axis 50 in FIG. 2 are the same as described in the embodiment in FIG. 1. This also applies to the embodiments from FIG.

  In the embodiment of FIG. 2, the length of the tapered waveguides 22 to 25 can be about half the length of the tapered waveguides 20 and 21 of FIG. Thus, the length of the grating coupler can be suppressed to about 135 μm. Therefore, the length of the grating coupler can be further shortened compared with the embodiment of FIG.

  Similarly, when the number of branch elements is increased by one in the optical branching unit 300, the light incident from the waveguide is branched into eight, and the length of the taper waveguide is further halved. The length can be shortened to 112.5 μm, for example. Even if the optical branching portion remains in a two-stage configuration, the length of the grating coupler can be set to about 100 μm by appropriately setting the lengths of the tapered waveguides 22 to 25 and the optical branching elements 30 to 32. is there. Of course, the length of the grating coupler can be made smaller than 100 μm by configuring the optical branching unit 300 in a three-stage configuration and setting the lengths of other portions as appropriate.

  FIG. 3 is a schematic plan view showing the configuration of another embodiment of the grating coupler of the present invention. The difference from the embodiment of FIG. 2 is that two tapered waveguides (23 ′, 24 ′) with a narrower width are provided among the four tapered waveguides (22-25) constituting the tapered waveguide section 200. It is a point.

  With such a configuration, the intensity of light emitted from the tapered waveguide having a narrow width increases, so that the light intensity distribution in the width direction of the grating element 10 can be adjusted at least at the incident end of the grating section 100. As a result, the intensity distribution of the emitted light can be adjusted to a desired shape. If the grating is uniform also in the waveguide direction, since most of the light is emitted near the incident end of the grating section 100, a practically sufficient effect can be obtained with this configuration. Note that the length of the grating coupler can be shortened as in the embodiment of FIG.

  FIG. 4 is a schematic plan view showing the configuration of another embodiment of the grating coupler of the present invention. The difference from the embodiment of FIG. 2 is that the grating section 100 is composed of four grating elements 11 to 14 separated from each other. However, in this embodiment, in each of the four grating elements 11 to 14, each lattice position of the one-dimensional lattice is in a direction orthogonal to the direction in which the one-dimensional lattice has periodicity within a common range where the one-dimensional lattice exists. Are arranged so that

  When there is one one-dimensional grating coupler, the light intensity near the center tends to increase in the width direction of the one-dimensional grating. On the other hand, when the configuration as in the embodiment shown in FIG. 4 is adopted, for example, the intensity distribution of the light in the lateral width direction can be adjusted more evenly by the grating elements 11 to 14 separated from each other. The intensity distribution of emitted light can be adjusted to be more uniform. Since there is a gap between the adjacent grating elements 11 to 14, there is a concern that the amount of light decreases at that portion and the intensity distribution of the emitted light is slightly affected, but the gap width between the grating elements is reduced. If narrowed appropriately, it can be prevented from causing a problem in practice. Note that the length of the grating coupler can be shortened as in the embodiment of FIG.

  FIG. 5 is a schematic plan view showing the configuration of another embodiment of the grating coupler of the present invention. The difference from the embodiment of FIG. 4 is that the four grating elements 11 to 14 separated from each other of the grating section 100 have two different widths. However, in this embodiment, in each of the four grating elements 11 to 14, each lattice position of the one-dimensional lattice is in a direction orthogonal to the direction in which the one-dimensional lattice has periodicity within a common range where the one-dimensional lattice exists. It is the same as that of embodiment of FIG.

  With such a configuration, the light intensity distribution in the lateral width direction is achieved by the grating elements 11 to 14 separated from each other, together with the function and effect of the two tapered waveguides (23 ′, 24 ′) with the taper narrowed. Can be further adjusted, and the intensity distribution of emitted light can be further adjusted to a desired form. In particular, when light is emitted not only near the input end of the grating unit 100 but also to a certain distance toward the output end side, the light intensity distribution in the lateral width direction can be kept constant over the entire radiation region. it can. Since there is a gap between adjacent grating elements 11 to 14, there is a concern that the amount of light is reduced at that portion and the intensity distribution of the emitted light is slightly affected, but the gap width between the grating elements, etc. If it is set appropriately narrow, it can be prevented from causing a problem in practice. Note that the length of the grating coupler can be shortened as in the embodiment of FIG.

  FIG. 6 is a schematic plan view showing the configuration of another embodiment of the grating coupler of the present invention. In the present embodiment, each grating position of the one-dimensional grating is aligned in a direction orthogonal to the direction in which the one-dimensional grating has periodicity within the common range where the one-dimensional grating exists in the two grating elements 15 and 16. It is constituted as follows. The length of the grating coupler can be shortened as in the embodiment of FIG.

  The embodiment shown in FIG. 6 differs from the embodiment shown in FIG. 1 in two points. The first difference is that the grating section 100 is composed of two grating elements 15 and 16 separated from each other. That is. As described above, the spread of light in the lateral width direction can be adjusted by the grating elements 15 and 16 separated from each other, and the intensity distribution of the emitted light can be adjusted to a desired shape. Since there is a gap between adjacent grating elements 15 and 16, there is a concern that the amount of light decreases at that portion and the intensity distribution of emitted light is slightly affected. If narrowed appropriately, it can be prevented from causing a problem in practice.

The second difference between the embodiment of FIG. 6 and the embodiment of FIG. 1 is that the distance from the connection position with the tapered waveguide to the start position of the grating differs between the grating elements 15 and 16. By adopting such a configuration, it becomes possible to reduce or cancel the influence of the return light P _ref (see FIG. 7A) from each grating element in the waveguide direction. In particular, the difference in distance from the connecting position of the grating elements 15 and 16 to the starting position of the grating is (2n + 1) λ / 4 (where n is an integer greater than or equal to 0 and λ is the wavelength of light). Since the phases of the return light from the grating element 15 and the return light from the grating element 16 are shifted by 180 °, they will cancel each other in the process of being combined by flowing backward through the optical branching element 30. The influence of the return light P _ref is canceled out.

  In the embodiment shown in FIGS. 1 to 6, at least one of the period of the one-dimensional grating, the depth of the unevenness, the layer structure, and the refractive index of the medium of the grating unit 100 is modulated in a direction having periodicity. You can also By adopting such a configuration, a grating pattern that is not uniform in the direction having periodicity can be realized in the grating section 100, and the shape of the mode field can be easily optimized.

  Also, in the embodiment shown in FIGS. 1 to 6, nothing is connected to the side opposite to the input end of the grating unit 100. For example, in order to collect transmitted light, a taper unit or an optical multiplexing unit is newly added. (The same structure as that of the optical branching unit and operated in the opposite direction) may be connected, or bidirectional operation may be performed with such a structure.

  The shape of the waveguide is not limited to the thin wire waveguide, but may be other forms such as a rib-type waveguide or a combination thereof.

  The embodiments of the present invention have been described above with reference to the drawings. However, those skilled in the art can use other similar embodiments, and the embodiments can be appropriately configured without departing from the present invention. It should be noted that changes or additions can be made.

10, 11, 12, 13, 14; grating elements 20, 21, 22, 23, 23 ', 24, 24', 25, 720; tapered waveguides 30, 31, 32; optical branching elements 100, 710; 200; taper waveguide part 300; optical branching part 701; substrate 702; BOX layer 714; core layer 716;

Claims (8)

A grating coupler formed on a substrate, the grating coupler comprising a grating portion, a tapered waveguide portion, and an optical branching portion,
The grating section includes one grating element or a plurality of grating elements arranged adjacent to each other in parallel. Each of the grating elements includes a one-dimensional grating, and each one-dimensional grating included in the plurality of grating elements includes Have periodicity in the same or substantially the same direction as each other,
The tapered waveguide portion includes more tapered waveguides than the number of grating elements;
The optical branching unit includes one input end and a plurality of output ends at least as many as the tapered waveguide,
Each of the grating elements is connected to one end on the wide side of the tapered waveguide, or two or more in parallel,
One of the plurality of output ends of the optical branching portion is connected to one end of each tapered waveguide on the narrow side,
A grating coupler characterized by that.   The one-dimensional grating is orthogonal to the direction in which the one-dimensional grating has periodicity, excluding the gap between adjacent grating elements, over the range of the length of the one-dimensional grating that is the shortest in the direction having periodicity among the one-dimensional gratings of all grating elements. The grating coupler according to claim 1, wherein the lattice positions of the one-dimensional lattice are aligned in a direction to be aligned.   The grating coupler according to claim 1, wherein the optical branching unit includes a multimode interferometer.   The grating coupler according to any one of claims 1 to 3, wherein the grating coupler has an axis of line symmetry.   The said grating part contains three or more grating elements, The width of the direction orthogonal to the direction in which those one-dimensional grating | lattices have periodicity has two or more types, The any one of Claim 1 thru | or 4 characterized by the above-mentioned. The grating coupler described in the paragraph.   The grating coupler according to claim 1, comprising a plurality of grating elements having different distances from a connection position with the tapered waveguide to a starting position of the grating.   The grating coupler according to any one of claims 1 to 4, wherein the number of grating elements is one.   The at least one of the period of the one-dimensional grating of each grating element of the grating part, the depth of the unevenness, the layer structure, and the refractive index of the medium is modulated in a direction having periodicity. 8. The grating coupler according to any one of 7 above.

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